The invention relates to a circuit configuration for two-wire/four-wire conversion. The circuit configuration has a receiver which is coupled on an input side to a digital two-wire reception path via which its receives a digital signal. A signal-processor is connected downstream of the receiver and emits a first and second signal. An echo-canceling device receives the first signal and emits a third signal. A digital-to-analog converter is provided which receives the second signal and emits a fourth signal. A hybrid circuit is coupled to an analog four-wire transmission-reception path, receives the fourth signal, and emits a fifth signal. An analog-to-digital converter receives the fifth signal and emits a sixth signal. A transmitter receives the sixth signal and is coupled on an output side to a digital two-wire transmission path and emits a digital transmitted signal to the latter.
Such circuit configurations are known, for example from Published, European Patent Application EP 0 122 594 A, U.S. Pat. No. 5,051,981 and in multifarious configurations, for example from International Patent Application WO 95/17049. In this case, use is made, in particular, of a method for reducing clipping or quantization effects of digital-to-analog converters in the transmission path of the echo-canceling unit. Systems particularly suitable for ADSL, which use multi-carrier modulation, are also described, inter alia, in John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. pages 5-14, May 1990, in S. Fleming et al., Telephony, Jul. 12, 1993, pages 20-26 and in M. Ho et al., 1993 International Conference on Communications, pages 772-776, May 1993. One problem with these three last-named configurations is that noise caused by clipping and quantization cannot be eliminated by conventional echo-canceling systems, for which reason the transmission properties of the transmission system are negatively affected. These effects are substantially reduced with the aid of the method and transmission systems presented in International Patent Application WO 95/17049. However, in this case increased demands are made, in particular, on the quality of the digital-to-analog conversion. The digital-to-analog conversion should be virtually ideal in this case, and so digital-to-analog converters that do not meet the Nyquist criterion are unsuitable.
However, suitable digital-to-analog converters require both a high outlay in terms of circuitry, and a high outlay on compensation, for which reason they are less suitable for implementation in integrated circuit technology. By contrast, digital-to-analog converters operating according to the sigma-delta modulator principle are very well suited for implementation in integrated circuit technology. However, these converters do not meet the Nyquist criterion, and therefore cannot be used straight away in the systems described in International Patent Application WO 95/17049. A suitable digital-to-analog converter is known, for example, from U.S. Pat. No. 5,585,802.
It is accordingly an object of the invention to provide a circuit configuration for two-wire/four-wire conversion which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which can be more effectively integrated even in the case of the use of the known method for reducing clipping and quantization effects.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for two-wire/four-wire conversion, including:
a receiver having an input side to be coupled to a digital two-wire reception path and receiving a digital signal;
a signal-processor connected downstream of the receiver and emitting a first signal and a second signal;
an echo-cancellation unit receiving the first signal and emitting a third signal;
a digital-to-analog converter receiving the second signal and emitting a fourth signal, the digital-to-analog converter having an interpolation filter receiving the second signal, a digital noise-shaping filter connected downstream of the interpolation filter, and an analog reconstruction filter connected in turn, downstream, of the noise-shaping filter, the digital noise-shaping filter generating a seventh signal;
a hybrid circuit to be coupled to an analog four-wire transmission-reception path and receiving the fourth signal, the hybrid circuit emitting a fifth signal;
an analog-to-digital converter receiving the fifth signal and emitting a sixth signal;
an approximation filter connected downstream of the digital noise-shaping filter and receiving the seventh signal and generating a filtered output signal;
an echo-estimating filter disposed downstream of the approximation filter and receiving the filtered output signal from the approximation filter and outputting an output signal;
a subtractor receiving the output signal from the echo-estimating filter being subtractively superimposed with the sixth signal and outputting a transmit signal; and
a transmitter receiving the transmit signal, having an output side to be coupled a digital two-wire transmission path, and emitting a digital transmitted signal to the digital two-wire transmission path.
The circuit configuration has a sigma-delta modulator as the digital-to-analog converter. In particular, the digital-to-analog converter contains as the sigma-delta modulating device the interpolation filter, to which the second signal is fed, the digital noise-shaping filter connected downstream of the interpolation filter, and the analog reconstruction filter connected, in turn, downstream of the noise-shaping filter. The fourth signal can be tapped in this care at the output of the analog reconstruction filter. The second signal is digitally interpolated in the interpolation filter and thus conditioned for the following oversampling. The downstream noise-shaping filter generates from the N bit of the digital signal at the output of the interpolation filter a 1-bit wide pulse modulation signal with the use of a plurality of, for example 1 to 6, feedback loops. The analog signal is reconstructed from the 1-bit wide digital bit stream by an analog filter such as, for example, an RC circuit or a switch-capacitor network.
According to the invention, the output signal of the digital noise-shaping filter is fed to the approximation filter downstream of which, in turn, the echo-estimating filter is connected. The output signal of the echo-estimating filter is in this case subtractively superimposed on the output signal of the analog-to-digital converter, specifically the sixth signal. The approximation filter serves the purpose of approximating the analog reconstruction filter. A DAC estimation filter is therefore involved, inter alia. Therefore, the analog signal at the output of the analog reconstruction filter, specifically the fourth signal, is approximated by the digital signal at the output of the approximation filter. Moreover, the oversampled signal in the approximation filter is reduced to the sampling rate used in the overall circuit configuration. The behavior of the hybrid circuit and of the analog transmission-reception path are approximated adaptively by the echo-estimating filter. It is advantageous in this case that both linear signals and nonlinear signals, produced during digital noise shaping, for example, are compensated.
Moreover, the above-mentioned development of the invention can be extended by a delay-estimating filter, to which, on the input side, the second signal, is fed, and whose output signal is subtractively superimposed on the output signal of the approximation filter with the output signal of the approximation filter and is fed together therewith to the echo-estimating filter. The result of this is that only the nonlinear signal component produced in the digital noise-shaping filter is passed on and, consequently, that the echo-estimating filter can be implemented in a simpler way and thus with a lower outlay. The delay-estimating filter is constructed in this case in such a way that the time delay and the amplitude response of the interpolation filter are compensated.
In another development of the invention, a dedicated digital decimation filter that is connected downstream of the approximation filter is used for the purpose of reducing the sampling rate. The echo-estimating filter is consequently located downstream of the digital decimation filter. In accordance with the development, the output signal of the interpolation filter is subtractively superimposed on the output signal of the approximation filter, and thus fed to the digital decimation filter. By splitting up the approximation filter into a purely approximating (xe2x80x9cestimatingxe2x80x9d) part and into a decimation part, the subtraction is carried out at high sampling rates, it being possible to omit compensation of the delay time.
Furthermore, it is also possible to provide downstream of the digital decimation filter a hybrid compensation unitxe2x80x94also known as hybrid estimation filterxe2x80x94which approximates the nonlinearities, for example of the line drivers in the hybrid circuit. The approximation is performed, for example, via a nonlinear characteristic that is applied to the digital bit stream at the output of the digital decimation filter. Furthermore, the output signal of the echo-estimating filter can be combined in series with the output signal of the analog-to-digital converter with the interposition of the further interpolation filter, the further digital noise-shaping filter and the further decimation filter. The configuration composed of the further interpolation filter, the further digital noise-shaping filter and the further decimation filter serves to simulate the transmission properties of the analog-to-digital converter.
Moreover, it can be provided that the signal-processor additionally emits a seventh signal which is fed to the transmitter in a fashion subtractively combined with the sixth signal, with the interposition of the echo estimation unit, and/or that the signal-processor additionally emit an eighth signal which is provided for controlling the echo-cancellation unit.
Finally, further signal processors are connected between the analog-to-digital converter and the transmitter.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for two-wire/four-wire conversion, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.